This invention relates to adjustable power control for a microwave magnetron of the type used in a microwave oven.
The microwave oven is the familiar appliance used to heat or cook foods by exposure of the food to microwave energy radiation. For this purpose conventional microwave ovens employ an electronic vacuum tube known in the art as a magnetron. Simply stated the magnetron is a device having unidirectional carrying characteristics which converts DC voltage and current into energy of the microwave frequency range such as for example, 2,450 megahertz. That DC voltage and current is provided by the power supply which converts normal household line voltage typically 120 or 240 volts AC to the normal operating voltages on the order of 3,000 to 4,000 volts DC required in the operation of existing microwave magnetrons. In its essentials present microwave oven power supplies contain a transformer for stepping-up the 120 volt AC to the level of less than the required 3,000 to 4,000 volts, a voltage doubler-rectifier which provides the required DC voltage for the magnetron and a source of low voltage for the heater of the magnetron.
As is known microwave energy generated by the magnetron is taken from the magnetron output and transmitted either directly or indirectly into the oven chamber. The average power supplied to the magnetron is set within limits by the design of both the power supply and the magnetron and is generally directly related to the microwave output power thereby generated by the magnetron. It is known that adjustment of the microwave power can be made within limits by adjustment of the DC voltage and, hence, current level to the magnetron. Most microwave ovens in commercial use contain power supplies that employs a high leakage reactance transformer in combination with a modified half-wave voltage doubler known also as the villard circuit to rectify and double the voltage output of the high voltage transformer, and apply a high voltage DC to the magnetron. Examples of such circuits appear in patents to Fineburg U.S. Pat. No. 3,396,342. U.S. Pat. No. 3,651,317 to Tingley and U.S. Pat. No. 3,684,978 to Otaguro which have been made known to applicants.
Recent practice is to provide additional elements within the oven supply that permit the microwave oven user to adjust the average power of the magnetron as offering high or low power or a mode to permit adjustment to any power level. One example of the high to low power adjustment in this combination is illustrated in the aforesighted patent to Otagro U.S. Pat. No. 3,684,978 in which the series capacitance is changed from one value to another. Still another method is to employ a semi-conductor controlled switching device such as the bidirectional triax in the primary circuit of the transformer so as to initiate current in the transformer primary to thereby regulate the average amount of current into the power supply which arrangement is found in the Litton Microwave Ovens sold under the brand name "Vari-cook." It is noted that in the Vari-Cook Oven circuit a separate filament or heater transformer is required because of the limited current into the primary of the high voltage transformer and the expedient of having the filament winding combined as a separate winding upon the high voltage transformer cannot presently be employed. Additionally the use of pulse techniques inherent in the Vari-Cook structure to the primary winding of the transformer creates additional voltage stresses on the transformer insulation that are best avoided. Another method of controlling the current appears described in U.S. Pat. No. 3,760,291 to Levinson in which a variable resistor is included in the current path to the magnetron so that the magnetron operating current is varied as a function of level of resistance as has been made known to us. This circuit appears impractical in that rather expensive resistors are employed which consumes current and generates heat.
The present invention relates to the control of the average power output of the oven magnetron by controlling the voltage in the secondary side of the transformer. More particularly the invention provides a simple control that allows the user to selectively adjust the power level of a magnetron to a certain range. In so doing the filament voltage may be supplied by the simple winding on the same transformer structure that contains the high voltage winding. The primary pulse technique of the Vari-Cook structure is avoided improving reliability of the transformer in our opinion, and any line surges caused by lightning on the input line as might destroy semi-conductor type control devices connected in the primary circuit are believed minimized.